Re: [PATCH] PPC assembly implementation of SHA1

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Sat, 2005-04-23 at 12:42 +0000, linux@horizon.com wrote:

> - You don't need to decrement %r1 before saving registers.
>   The PPC calling convention defines a "red zone" below the
>   current stack pointer that is guaranteed never to be touched
>   by signal handlers or the like.  This is specifically for
>   leaf procedure optimization, and is at least 224 bytes.

On ELF ppc32 ABI ? Hrm... the SysV ABI document says

"The only permitted references with negative offsets from the stack
pointer are those described here for establishing a stack frame."

I know MacOS had a red zone, but do we ?

> - Is that many stw/lwz instructions faster than stmw/lmw?
>   The latter is at least more cahce-friendly.

More cache friendly ? Hrm.. I wouldn't be sure. Also, I remember readind
a while ago that those store/load multiple instructions aren't that
recommended. They aren't very good on some CPU models. I would expect
the bus/cache bandwidth to be the limiting factor here anyway, and as
you point out below, the they don't deal with unaligned access very well
in all cases.